The present invention relates to reinforced composites, particularly to such composites comprising two or more layers of a fiber-reinforced crosslinked epoxy resin, wherein the adjacent epoxy resin layers are interspaced with a layer of a thermoplastic resin, to the preparation of said composites and to their use.
Reinforced composites or composite materials have, in addition to many excellent properties such as high specific strength, engineerability and low energy content, a number of limitations. As these fiber-reinforced composite materials are generally characterized by a high stiffness, bending of such composites will induce high shear stresses in planes parallel to the principal fiber direction. When such composites are submitted to impact testing these materials show an unacceptable degree of delamination. It is generally understood that this problem is related to the insufficient ability of these composite materials to dissipate the impact energy.
Methods which have been proposed to improve the overall performance of the fiber-reinforced composites include increasing the damage-tolerance of said composites by reducing the crosslink density of the polymer matrix or by the incorporation of an elastomeric compound. However these methods do not generally provide the desired results.
Another known modification for said reinforced composites comprises the replacement of the thermoset matrix with a thermoplastic one. Although said method improves the strain resistance of the polymer matrix, compression strength is simultaneously reduced to an unacceptably low level, due to the generally lower stiffness of the thermoplastic matrix.
Another method for improving the overall performance of the fiber-reinforced crosslinked epoxy resin composites comprises the insertion of a thermoplastic film, such as one made from Nylon 6, between the different layers of reinforced epoxy resin during the preparation of said composites. However, with this method it has been observed that the performance is hampered by insufficient adhesion between the thermoplastic interlayer and the thermoset matrix.
There remains a considerable need to improve the overall performance of fiber-reinforced, crosslinked epoxy resin composites. The problem underlying the present invention is to develop a fiber-reinforced crosslinked epoxy resin based composite, which does not suffer from one or more of the above-mentioned problems, i.e. a composite having an improved overall performance.